The advantages in durability and ease of engagement which result from chamfering the sharp edges of the teeth of gears, splines and the like have been well appreciated for a number of years. The early tedious methods of chamfering by hand, such as with a file, have been overcome with the introduction of specialized machines adapted for such purpose.
There are of course a number of conceivable machines which can be used to provide the uniform chamfer which is desired on the edges of gear teeth. In some instances the desired beveling has been achieved by cold working. More perferably, the chamfering is obtained by machining, such as those which utilize the circular motion of a planar faced cutting tool. One machine generally of the type to which the invention is pertinent is shown in U.S. Pat. No. 3,286,593 to Bibbens. Another machine, mentioned below, is called a Sheffield machine. Such machines are adapted to provide the cutting tool with reciprocating arcuate motion, whereby a chamfer is made on each forward stroke, and the gear is indexed during each return stroke to present a new tooth form to the cutter. The present invention is specifically adapted to such types of machines wherein the tool swings through an arc in a plane which is generally perpendicular to the plane of the sides of the teeth being chamfered. Since gear teeth have involute shapes of some complexity, and since the motion of the cutting tool has a transverse circular motion, the geometry of the tool is not at all evident. Accordingly, the making of cutting tools for chamfering machines has remained within the realm of a skilled tool and die maker. Such toolmaking skill is acquired by example from their predecessors and from their own experience and judgment. Essentially, a "cut and try" procedure is used. That is, the craftsman relies on his experience to make a first tool. For example, a 45 degree projection of the tooth form might be used, since the tangent to the arcuate motion of the cutting tool with respect to the plane of the teeth ends approximately lies on such an incline. The toolmaker then places the tool in an actual machine and tests its cutting action on an actual gear. After observing the results according to imperfections of the chamfer which is produced, he repetitively changes the contour of the tool until he arrives at a tool which hopefully produces the desired specification chamfer.
In precision and high performance machinery, the specification on chamfer width can be relatively tight. For example, a chamfer on a typical gear of about 11 inch diameter and having 69 teeth, might require the chamfer width of 0.040.+-.0.010. Typically, the chamfer is placed on the gear prior to hardening. When the gear is subsequently finished by further material removal, a comparatively small chamfer remains. Thus the location of the chamfer must be accurately placed to achieve the desired purpose. Despite the ability of skilled mechanics, the making of a tool is a tedious and labor intensive process. Actual gears must be made before the tool shape can be ascertained, leading to production delays. Valuable gear parts can be consumed in the trial and error tool-making process. And perfection is seldom achieved; some inferiority in the quality of the chamfer must often be accepted in the interests of expediency and economy.